A position sensor is disclosed in German Auslegeschrift 1,773,303. In this known apparatus, a core is surrounded by an exciting coil, which is coiled uniformly about the core and which is energized by a periodic voltage or a periodic current, so that the core is driven far enough into the saturation range. Four receiving coils are disposed over the circumference, offset in each case by 90.degree., the diametrically opposed receiving coils being connected back-to-back. By forming the difference between the voltages induced in the pairs of receiving coils, the transformed voltage basically is cancelled in these coils. However, an external magnetic field, particularly the horizontal component of the earth's magnetic field, brings about a shift in the operating point on the characteristic magnetization curve, which results in a phase shift that causes a non-cancelling voltage portion. This consists essentially of harmonics of an even order of the exciting frequency, the amplitudes of which are proportional to the external magnetic field. In the evaluating circuit, which is connected with the receiving coil, the second harmonic advisably is evaluated in order to generate a signal which corresponds to the direction of the external magnetic field. As a result of the inhomogeneities and asymmetries in the core and in the coils, this apparatus is not suitable for more accurate measurements. Moreover, it has a plurality of components and is therefore quite expensive. Even in the case of a variation with an annular magnetic core floating in a hermetically sealed toroid, said toroid is supported on a quite expensive suspension device.
French publication 2,601,185 discloses a magnetic core for a compass, in which the receiver coils embrace the annular magnetic core diametrically and do not embrace the cross section of the annular magnetic core, as they do in the aforementioned German Auslegeschrift 1,773,303.
German Offenlegungsschrift 3,012,241 discloses a further apparatus for measuring a magnetic field. For this apparatus, a liquid-supported float is used, which can be pivoted on a bearing shaft lying in a horizontal plane. The bearing shaft rests in an orbiting guiding groove, so that the float can also carry out a rotational movement. A rigid assignment of the float to a particular orientation of the housing is thus not assured.